Tantalum pentoxide, also known astantalum(V) oxide, is theinorganic compound with theformulaTa 2O 5. It is a white solid that is insoluble in all solvents but is attacked by strong bases andhydrofluoric acid.Ta 2O 5 is an inert material with a highrefractive index and low absorption (i.e. colourless), which makes it useful for coatings.[2] It is also extensively used in the production ofcapacitors, due to its highdielectric constant.
Tantalum occurs in the mineralstantalite andcolumbite (columbium being an archaic name for niobium), which occur inpegmatites, an igneous rock formation. Mixtures of columbite and tantalite are calledcoltan. Tantalum was discovered in Tantalite in 1802 byAnders Gustaf Ekeberg atYtterby, Sweden, and Kimoto, Finland. The mineralsmicrolite andpyrochlore contain approximately 70% and 10% Ta, respectively.
Tantalum ores often contain significant amounts ofniobium, which is itself a valuable metal. As such, both metals are extracted so that they may be sold. The overall process is one ofhydrometallurgy and begins with aleaching step; in which the ore is treated withhydrofluoric acid andsulfuric acid to produce water-solublehydrogen fluorides, such as theheptafluorotantalate. This allows the metals to be separated from the various non-metallic impurities in the rock.
The tantalum and niobium hydrogenfluorides are then removed from theaqueous solution byliquid-liquid extraction usingorganic solvents, such ascyclohexanone ormethyl isobutyl ketone. This step allows the simple removal of various metal impurities (e.g. iron and manganese) which remain in the aqueous phase in the form offluorides. Separation of the tantalum and niobium is then achieved bypH adjustment. Niobium requires a higher level of acidity to remain soluble in the organic phase and can hence be selectively removed by extraction into less acidic water.The pure tantalum hydrogen fluoride solution is then neutralised with aqueousammonia to givehydrated tantalum oxide (Ta2O5(H2O)x), which iscalcinated to tantalum pentoxide (Ta2O5) as described in these idealized equations:[3]
Tantalum oxide is frequently used in electronics, often in the form ofthin films. For these applications it can be produced byMOCVD (or related techniques), which involves thehydrolysis of its volatilehalides oralkoxides:
At least 2polymorphs are known to exist. A low temperature form, known as L- or β-Ta2O5, and the high temperature form known as H- or α-Ta2O5. The transition between these two forms is slow and reversible; taking place between 1000 and 1360 °C, with a mixture of structures existing at intermediate temperatures.[5] The structures of both polymorphs consist of chains built from octahedral TaO6 and pentagonal bipyramidal TaO7 polyhedra sharing opposite vertices; which are further joined by edge-sharing.[6][7] The overall crystal system isorthorhombic in both cases, with thespace group of β-Ta2O5 being identified asPna2 by single crystal X-ray diffraction.[8][9]
A high pressure form (Z-Ta2O5) has also been reported, in which the Ta atoms adopt a 7 coordinate geometry to give amonoclinic structure (space group C2).[10]
Purely amorphous tantalum pentoxide has a similar local structure to the crystalline polymorphs, built from TaO6 and TaO7 polyhedra, while the molten liquid phase has a distinct structure based on lower coordination polyhedra, mainly TaO5 and TaO6.[11]
The difficulty in forming material with a uniform structure has led to variations in its reported properties. Like many metal oxides Ta2O5 is aninsulator and itsband gap has variously been reported as being between 3.8 and 5.3 eV, depending on the method of manufacture.[12][13][14] In general the moreamorphous the material the greater its observed band gap.These observed values are significantly higher than those predicted bycomputational chemistry (2.3 - 3.8 eV).[15][16][17]
Itsdielectric constant is typically about 25[18] although values of over 50 have been reported.[19] In general tantalum pentoxide is considered to be ahigh-k dielectric material.
Ta2O5 does not react appreciably with either HCl or HBr, however it will dissolve inhydrofluoric acid, and reacts withpotassium bifluoride and HF according to the following equation:[20][21]
Owing to its highband gap anddielectric constant, tantalum pentoxide has found a variety of uses in electronics, particularly intantalum capacitors. These are used inautomotive electronics, cell phones, and pagers, electronic circuitry; thin-film components; and high-speed tools. In the 1990s, interest grew in the use of tantalum oxide as ahigh-k dielectric forDRAM capacitor applications.[22][23]
It is used in on-chip metal-insulator-metal capacitors for high frequencyCMOS integrated circuits. Tantalum oxide may have applications as the charge trapping layer fornon-volatile memories.[24][25] There are applications of tantalum oxide inresistive switching memories.[26]
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^Anthony Agulyanski (2004). "Fluorine chemistry in the processing of tantalum and niobium". In Anatoly Agulyanski (ed.).Chemistry of Tantalum and Niobium Fluoride Compounds (1st ed.). Burlington: Elsevier.ISBN978-0-08-052902-8.
^abAskeljung, Charlotta; Marinder, Bengt-Olov; Sundberg, Margareta (1 November 2003). "Effect of heat treatment on the structure of L-Ta2O5".Journal of Solid State Chemistry.176 (1):250–258.Bibcode:2003JSSCh.176..250A.doi:10.1016/j.jssc.2003.07.003.
^Stephenson, N. C.; Roth, R. S. (1971). "Structural systematics in the binary system Ta2O5–WO3. V. The structure of the low-temperature form of tantalum oxide L-Ta2O5".Acta Crystallographica Section B.27 (5):1037–1044.Bibcode:1971AcCrB..27.1037S.doi:10.1107/S056774087100342X.
^Hummel, Hans-U.; Fackler, Richard; Remmert, Peter (1992). "Tantaloxide durch Gasphasenhydrolyse, Druckhydrolyse und Transportreaktion aus 2H-TaS2: Synthesen von TT-Ta2O5 und T-Ta2O5 und Kristallstruktur von T-Ta2O5".Chemische Berichte.125 (3):551–556.doi:10.1002/cber.19921250304.
^Zibrov, I. P.; Filonenko, V. P.; Sundberg, M.; Werner, P.-E. (1 August 2000). "Structures and phase transitions of B-Ta2O5 and Z-Ta2O5: two high-pressure forms of Ta2O5".Acta Crystallographica Section B.56 (4):659–665.doi:10.1107/S0108768100005462.PMID10944257.S2CID22330435.
^Kukli, Kaupo; Aarik, Jaan; Aidla, Aleks; Kohan, Oksana; Uustare, Teet; Sammelselg, Väino (1995). "Properties of tantalum oxide thin films grown by atomic layer deposition".Thin Solid Films.260 (2):135–142.Bibcode:1995TSF...260..135K.doi:10.1016/0040-6090(94)06388-5.
^Fleming, R. M.; Lang, D. V.; Jones, C. D. W.; Steigerwald, M. L.; Murphy, D. W.; Alers, G. B.; Wong, Y.-H.; van Dover, R. B.; Kwo, J. R.; Sergent, A. M. (1 January 2000). "Defect dominated charge transport in amorphous Ta2O5 thin films".Journal of Applied Physics.88 (2): 850.Bibcode:2000JAP....88..850F.doi:10.1063/1.373747.
^Murawala, Prakash A.; Sawai, Mikio; Tatsuta, Toshiaki; Tsuji, Osamu; Fujita, Shizuo; Fujita, Shigeo (1993). "Structural and Electrical Properties of Ta2O5 Grown by the Plasma-Enhanced Liquid Source CVD Using Penta Ethoxy Tantalum Source".Japanese Journal of Applied Physics.32 (Part 1, No. 1B):368–375.Bibcode:1993JaJAP..32..368M.doi:10.1143/JJAP.32.368.S2CID97813703.
^Ramprasad, R. (1 January 2003). "First principles study of oxygen vacancy defects in tantalum pentoxide".Journal of Applied Physics.94 (9):5609–5612.Bibcode:2003JAP....94.5609R.doi:10.1063/1.1615700.
^Nashed, Ramy; Hassan, Walid M. I.; Ismail, Yehea; Allam, Nageh K. (2013). "Unravelling the interplay of crystal structure and electronic band structure of tantalum oxide (Ta2O5)".Physical Chemistry Chemical Physics.15 (5):1352–7.Bibcode:2013PCCP...15.1352N.doi:10.1039/C2CP43492J.PMID23243661.
^Macagno, V.; Schultze, J.W. (1 December 1984). "The growth and properties of thin oxide layers on tantalum electrodes".Journal of Electroanalytical Chemistry and Interfacial Electrochemistry.180 (1–2):157–170.doi:10.1016/0368-1874(84)83577-7.
^Hiratani, M.; Kimura, S.; Hamada, T.; Iijima, S.; Nakanishi, N. (1 January 2002). "Hexagonal polymorph of tantalum–pentoxide with enhanced dielectric constant".Applied Physics Letters.81 (13): 2433.Bibcode:2002ApPhL..81.2433H.doi:10.1063/1.1509861.
^Brauer, Georg (1965).Handbook of preparative inorganic chemistry. Academic Press. p. 256.ISBN978-0-12-395591-3.
^Ezhilvalavan, S.; Tseng, T. Y. (1999). "Preparation and properties of tantalum pentoxide (Ta2O5) thin films for ultra large scale integrated circuits (ULSIs) application - a review".Journal of Materials Science: Materials in Electronics.10 (1):9–31.doi:10.1023/A:1008970922635.S2CID55644772.
^Chaneliere, C; Autran, J L; Devine, R A B; Balland, B (1998). "Tantalum pentoxide (Ta2O5) thin films for advanced dielectric applications".Materials Science and Engineering: R.22 (6):269–322.doi:10.1016/S0927-796X(97)00023-5.
^Wang, X; et al. (2004). "A Novel MONOS-Type Nonvolatile Memory Using High-κ Dielectrics for Improved Data Retention and Programming Speed".IEEE Transactions on Electron Devices.51 (4):597–602.Bibcode:2004ITED...51..597W.doi:10.1109/TED.2004.824684.
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